An unclean spirit is simply a New Testament synonym, a more descriptive Jewish term, for a demon. The terms unclean spirit and demon seem to be interchangeable in Scripture. There is no clear difference in their definitions. Some translations refer to them as “impure spirits.”
Throughout the New Testament, the term unclean spirits (akathartos in the Greek language) is mentioned over twenty times. Throughout those passages we read that unclean spirits can possess people and cause them sickness and harm (Matthew 10:1; 12:43; Mark 1:26; Luke 4:36; 6:18; Acts 5:16; 8:7), that they are searching for someone to possess if they are not currently possessing someone (Matthew 12:43), that some are more unclean or evil than others (Luke 11:26), that unclean spirits can interact with one another (Mark 5:1–20; Matthew 12:45), and that unclean spirits are under God’s authority and must submit to Him (Mark 1:27; 3:11; 5:8, 13).
An unclean spirit or demon is “unclean” in that it is wicked. Evil spirits are not only wicked themselves, but they delight in wickedness and promote wickedness in humans. They are spiritually polluted and impure, and they seek to contaminate all of God’s creation with their filth. Their foul, putrid nature is in direct contrast to the purity and incorruption of the Holy Spirit’s nature. When a person is defiled by an unclean spirit, he takes pleasure in corrupt thoughts and actions; when a person is filled with the Holy Spirit, his thoughts and actions are heavenly.
Some people hold the idea that unclean spirits or demons are deceased humans who may or may not have been evil while alive. However, we know the unclean spirits mentioned in the Bible are not referring to the dead, for several reasons. One, humans are never called “spirits” when the word spirit is used as a stand-alone term, without a possessive. In Scripture, men are said to have a spirit/soul (saying “his spirit” in Proverbs 25:28 and 1 Corinthians 5:5), but men are not called “spirits.” Another reason is that, once a person dies, he immediately goes either to eternal life with the Lord or to eternal darkness in hell (Hebrews 9:27; 2 Corinthians 5:6–8; Matthew 25:46). Human spirits, therefore, do not and cannot wander on earth in their spirit bodies. Any unclean spirit that wanders around, taking up residence in places or people or interacting with people in any way, is a fallen angel—a demon (Matthew 12:44). All unclean spirits mentioned in Scripture are demons, and all demons are definitely unclean, unholy, impure, evil spirits doomed to an eternity in hell (Matthew 25:41).